The present invention relates to a fusion fixing apparatus employing heat and pressure and serving for fixing thermoplastic toner images. The apparatus generally comprises a heatable fixing roller with a polytetrafluoroethylene surface, a pressure roller which can be made to engage and disengage the fixing roller and which is moved from a rest position into engagement with the fixing roller by an actuating mechanism, and a wetting device for applying an anti-adhesive fluid to the fixing roller. The apparatus also comprises a driving mechanism for the rollers, which ensures that the rollers rotate at the same speed when they meet, so that the image carrier material carrying the toner images, which is positioned between the rollers, is transported without blurring the images.
In a known fusing apparatus of this type, disclosed in German Offenlegungsschriften No. 1,772,686 and No. 1,497,216, one being an application for a patent of addition to the other, the fixing roller provided with the polytetrafluoroethylene coating is heated from the inside. The pressure roller, which consists of a metal core carrying a jacket of vulcanized rubber which, in turn, is coated with a polytetrafluoroethylene coating, is caused to engage the fixing roller by an actuator. The driving mechanism for the two rollers comprises a one-way clutch which is connected with the pressure roller so that this roller can assume the same speed as the fixing roller when it meets the fixing roller. When the pressure roller is made to contact the fixing roller, the distance between the axle centers of the two rollers is reduced. The radius extending from the center of the fixing roller to its point of contact with the pressure roller remains constant because the fixing roller essentially consists of a rigid metal cylinder. The radius of the pressure roller extending from its center to the point of contact with the fixing roller is reduced by the pressure which must be applied to form a line of contact between the two rollers and ensure good fusion of the toner on the image carrier material. Upon passage of the image carrier material carrying the toner images, the imaged material is pressed against the surface of the fixing roller and possibly also against that of the pressure roller and is curved in accordance with the curvature of the fixing roller. By the natural curvature of this roller, the imaged material tends to adhere to the fixing roller during its passage through the rollers and thus interferes in an undesirable manner with the transport of the imaged material.
German Offenlegungsschrift No. 1,926,118 discloses a fixing apparatus for electrophotographic powder images in which heat and pressure are applied and which comprises a fixing roller which is heated from the outside by radiators. The fixing roller comprises several layers of silicon rubber and a toner-repellent layer, if desired, and it cooperates with a rigid pressure roller. The fixing roller possesses a certain flexibility and can be compressed when the rigid pressure roller is forced against it. The layers of silicon rubber may be replaced by highly temperature-resistant expanded rubber layers or by sealed elastic sleeves which may be made very flexible.
The heated fusion fixing apparatus disclosed in U.S. Pat. No. 3,988,817 also uses a rigid and a deformable roller. In this case, the fixing roller is rigid, whereas the pressure roller has an elastic layer which is applied to a metal core. A flexible sleeve which may consist, e.g., of fluoroethylene propylene, is pushed over this elastic layer. While the pressure roller rests against the fixing roller, a slight depression is formed in the pressure roller which corresponds to the curvature of the fixing roller surface.
These known devices have the disadvantage that a considerable compressive force is required to create a relatively large zone or area of contact between the fixing roller and the pressure roller. Due to the large expenditure on equipment which is thus required for supporting and driving the rollers, the entire driving system must be disassembled if a roller is to be replaced, which means that these devices are neither easy to service nor inexpensive.